Patterned magnetic recording disk with high bit-aspect-ratio and master mold for nanoimprinting the disk

ABSTRACT

A method for making a master mold that is used in the nanoimprinting process to make patterned-media disks with patterned data islands uses guided self-assembly of a block copolymer into its components. Conventional or e-beam lithography is used to first form a pattern of generally radial stripes on a substrate, with the stripes being grouped into annular zones or bands. A block copolymer material is then deposited on the pattern, resulting in guided self-assembly of the block copolymer into its components to multiply the generally radial stripes into generally radial lines. Various methods, including conventional lithography, guided self-assembly of a second block copolymer, and e-beam lithography, are then used to form concentric rings over the generally radial lines. After etching and resist removal, the master mold has a pattern of either pillars or holes, depending on the method used.

RELATED APPLICATIONS

This application is a Divisional of application Ser. No. 12/539,818filed Aug. 12, 2009, which is a Continuation-in-Part of application Ser.No. 12/141,062 filed Jun. 17, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to patterned-media magnetic recordingdisks, wherein each data bit is stored in a magnetically isolated dataisland on the disk, and more particularly to a method for making amaster mold to be used for nanoimprinting the patterned-media disks.

2. Description of the Related Art

Magnetic recording hard disk drives with patterned magnetic recordingmedia have been proposed to increase data density. In patterned media,the magnetic recording layer on the disk is patterned into smallisolated data islands arranged in concentric data tracks. To produce therequired magnetic isolation of the patterned data islands, the magneticmoment of spaces between the islands must be destroyed or substantiallyreduced to render these spaces essentially nonmagnetic. In one type ofpatterned media, the data islands are elevated regions or pillars thatextend above “trenches” and magnetic material covers both the pillarsand the trenches, with the magnetic material in the trenches beingrendered nonmagnetic, typically by “poisoning” with a material likesilicon (Si). In another type of patterned media, the magnetic materialis deposited first on a flat disk substrate. The magnetic data islandsare then formed by milling, etching or ion-bombarding of the areasurrounding the data islands. Patterned-media disks may be longitudinalmagnetic recording disks, wherein the magnetization directions areparallel to or in the plane of the recording layer, or perpendicularmagnetic recording disks, wherein the magnetization directions areperpendicular to or out-of-the-plane of the recording layer.

One proposed method for fabricating patterned-media disks is bynanoimprinting with a template or mold, sometimes also called a“stamper”, that has a topographic surface pattern. In this method themagnetic recording disk substrate with a polymer film on its surface ispressed against the mold. The polymer film receives the reverse image ofthe mold pattern and then becomes a mask for subsequent etching of thedisk substrate to form the pillars on the disk. In one type of patternedmedia, the magnetic layer and other layers needed for the magneticrecording disk are then deposited onto the etched disk substrate and thetops of the pillars to form the patterned-media disk. In another type ofpatterned media, the magnetic layers and other layers needed for themagnetic recording disk are first deposited on the flat disk substrate.The polymer film used with nanoimprinting is then pressed on top ofthese layers. The polymer film receives the reverse image of the moldpattern and then becomes a mask for subsequent milling, etching orion-bombarding the underlying layers. The mold may be a master mold fordirectly imprinting the disks. However, the more likely approach is tofabricate a master mold with a pattern of pillars corresponding to thepattern of pillars desired for the disks and to use this master mold tofabricate replica molds. The replica molds will thus have a pattern ofrecesses or holes corresponding to the pattern of pillars on the mastermold. The replica molds are then used to directly imprint the disks.Nanoimprinting of patterned media is described by Bandic et al.,“Patterned magnetic media: impact of nanoscale patterning on hard diskdrives”, Solid State Technology S7+Suppl. S, SEP 2006; and by Terris etal., “TOPICAL REVIEW: Nanofabricated and self-assembled magneticstructures as data storage media”, J. Phys. D: Appl. Phys. 38 (2005)R199-R222.

In patterned media, the bit-aspect-ratio (BAR) of the pattern or arrayof discrete data islands arranged in concentric tracks is the ratio oftrack spacing or pitch in the radial or cross-track direction to theisland spacing or pitch in the circumferential or along-the-trackdirection. This is the same as the ratio of linear island density inbits per inch (BPI) in the along-the-track direction to the trackdensity in tracks per inch (TPI) in the cross-track direction. The BARis also equal to the ratio of the radial dimension of the bit cell tothe circumferential dimension of the bit cell, where the data island islocated within the bit cell. The bit cell includes not only the magneticdata island but also one-half of the nonmagnetic space between the dataisland and its immediately adjacent data islands. The data islands havea ratio of radial length to circumferential width, referred to as theisland aspect ratio (JAR), that can be close to or greater than the BAR.

In patterned media, there are two opposing requirements relating to theBAR. The first requirement is that to minimize the resolutionrequirement for fabricating the islands, it is preferable that the arrayof islands have a low BAR (about 1). The second requirement is that toallow for a wider write head pole, which is necessary for achieving ahigh write field to allow the use of high coercivity media for thermalstability, it is preferable that the array of islands have a higher BAR(about 2 or greater). Also, the transition from disk drives withconventional continuous media to disk drives with patterned media issimplified if the BAR is high because in conventional disk drives theBAR is between about 5 to 10. Other benefits of higher BAR include lowertrack density, which simplifies the head-positioning servo requirements,and a higher data rate.

The making of the master template or mold is a difficult and challengingprocess. The use of electron beam (e-beam) lithography using a Gaussianbeam rotary-stage e-beam writer is viewed as a possible method to make amaster mold capable of nanoimprinting patterned-media disks with a BARof about 1 with a track pitch (island-to-island spacing in the radial orcross-track direction) of about 35 nm, and an island pitch(island-to-island spacing in the circumferential or along-the-trackdirection) of about 35 nm. If the data islands have a radial length andcircumferential width each of about 20 nm for an IAR of 1, then thesedimensions generally limit the areal bit density of patterned-mediadisks to about 500 Gbit/in². To achieve patterned-media disks with bothan ultra-high areal bit density (at least 1 Terabits/in²) and a higherBAR, a track pitch of 50 nm and an island pitch of about 12.5 nm will berequired, which would result in a BAR of 4. However, a master moldcapable of nanoimprinting patterned-media disks with an island pitch of12.5 nm over an area equal to the data area of a disk is not achievablewith the resolution of e-beam lithography.

What is needed is a master mold and a method for making it that canresult in patterned-media magnetic recording disks with both therequired high areal bit density and higher BAR (greater than 1 andpreferably about 2 or greater).

SUMMARY OF THE INVENTION

The present invention relates to a method for making a master mold thatis used in the nanoimprinting process to make patterned-media disks withan island pitch difficult to achieve with the resolution of e-beamlithography. The master mold may be used to directly nanoimprint thedisks, but more likely is used to make replica molds which are then usedto directly nanoimprint the disks. The method uses conventional ore-beam lithography to form a pattern of generally radial stripes on asubstrate, with the stripes being grouped into annular zones or bands. Ablock copolymer material is deposited on the pattern, resulting inguided self-assembly of the block copolymer into its components tomultiply the generally radial stripes into generally radial lines. Theradial lines preferably have a higher circumferential density than thatof the radial stripes. Various methods, including conventionallithography, guided self-assembly of a second block copolymer, ande-beam lithography, are then used to form concentric rings over thegenerally radial lines. The concentric rings are used to define theradial length of the islands formed by the master mold. After etchingand resist removal, the master mold has a pattern of either pillars orholes, depending on the method used. The pillars or holes are arrangedin circular rings, with the rings grouped into annular bands. Thespacing of the concentric rings is selected so that following theetching process the master mold has an array of pillars or holes withthe desired BAR, which is greater than 1, preferably about 2 or greater.Because the invention allows the circumferential density of the mastermold pillars or holes to be at least doubled from what could be achievedwith just e-beam lithography, the subsequently nanoimprintedpatterned-media disks can have both a high BAR (greater than 1 andpreferably equal to or greater than 2) and an ultra-high areal density.

A first embodiment of the method uses conventional optical or e-beamlithography to form concentric rings of resist over the generally radiallines of one of the block copolymer components. After etching to removeportions of the remaining block copolymer component between theconcentric rings, and removal of the resist, a pattern of pillars of theremaining block copolymer component is formed on the substrate. Thesepillars of remaining block copolymer component are used as an etch maskto pattern the substrate. After etching and removal of the pillars ofremaining block copolymer component, a master mold remains that haspillars of substrate material arranged in circular rings, with the ringsgrouped into annular bands.

A second embodiment of the method uses a first block copolymer materialwith bulk period L₀=L_(rad), resulting in guided self-assembly of thefirst block copolymer into its components to multiply the generallyradial stripes into generally radial lines of alternating first blockcopolymer components. The radial lines of one of the components areremoved, leaving the radial lines of the remaining component of thefirst block copolymer. A protective layer is then deposited over theradial lines of the remaining component of the first block copolymer toprevent their movement during subsequent processing. Then, a secondblock copolymer material with bulk period L₀=L_(circ) is deposited overthese radial lines to define generally circumferential rings. Thecircumferential rings of one of the components of the second blockcopolymer are removed, leaving the circumferential rings of theremaining component of the second block copolymer. The circumferentialrings of the remaining second block copolymer component and theunderlying radial lines of the remaining first block copolymer componentform a grid that functions as an etch mask. Etching of the substratethrough this mask, followed by removal of the remaining block copolymermaterial, results in a master mold with a pattern of recesses or holesarranged in circular rings, with the rings grouped into annular bands.The ratio of L_(circ)/L_(rad) defines the BAR for the disk made from themold.

A third embodiment of the method uses a block copolymer material withbulk period L₀, resulting in guided self-assembly of the block copolymerinto its components to multiply the generally radial stripes intogenerally radial lines of alternating block copolymer A and Bcomponents. Then an e-beam writer generates a high dose e-beam in apattern of concentric rings which cross-links the A and B copolymersexposed to the high dose e-beam, resulting in concentric rings formed ofcross-linked polymer material. The underlying radial lines of the Bcomponent are then removed, leaving the cross-linked concentric ringsand underlying radial lines of the A component. This structure thenserves as an etch mask to pattern recesses or holes into the underlyingsubstrate.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a top view of a disk drive with a patterned-media type ofmagnetic recording disk as described in the prior art.

FIG. 2 is a top view of an enlarged portion of a patterned-media type ofmagnetic recording disk showing the detailed arrangement of the dataislands in one of the bands on the surface of the disk substrate.

FIG. 3 is a side sectional view of one type of a patterned-media diskshowing the data islands as elevated, spaced-apart pillars that extendabove the disk substrate surface with trenches between the pillars.

FIG. 4 is a schematic view of a patterned-media disk showing a patternof radial lines in three annular bands, with each radial line meant torepresent data islands from all the concentric tracks in the band.

FIGS. 5A-5N are views of a small portion of one annular band of themaster mold at successive stages of a first embodiment of the method ofmaking the master mold according to the present invention.

FIG. 5O is a scanning electron microscope (SEM) micrograph of a portionof the master mold made according to the method shown in FIGS. 5A-5N.

FIGS. 6A-6H are views of a small portion of one annular band of themaster mold at successive stages of a second embodiment of the method ofmaking the master mold according to the present invention.

FIGS. 7A-7D are views of a small portion of one annular band of themaster mold at successive stages of a third embodiment of the method ofmaking the master mold according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a top view of a disk drive 100 with a patterned magneticrecording disk 10 as described in the prior art. The drive 100 has ahousing or base 112 that supports an actuator 130 and a drive motor forrotating the magnetic recording disk 10 about its center 13. Theactuator 130 may be a voice coil motor (VCM) rotary actuator that has arigid arm 134 and rotates about pivot 132 as shown by arrow 124. Ahead-suspension assembly includes a suspension 121 that has one endattached to the end of actuator arm 134 and a head carrier 122, such asan air-bearing slider, attached to the other end of suspension 121. Thesuspension 121 permits the head carrier 122 to be maintained very closeto the surface of disk 10. A magnetoresistive read head (not shown) andan inductive write head (not shown) are typically formed as anintegrated read/write head patterned on the trailing surface of the headcarrier 122, as is well known in the art.

The patterned magnetic recording disk 10 includes a disk substrate 11and discrete data islands 30 of magnetizable material on the substrate11. The data islands 30 function as discrete magnetic bits for thestorage of data and are arranged in radially-spaced circular tracks 118,with the tracks 118 being grouped into annular bands 119 a, 119 b, 119c. The grouping of the data tracks into annular zones or bands permitsbanded recording, wherein the angular spacing of the data islands, andthus the data rate, is different in each band. In FIG. 1, only a fewislands 30 and representative tracks 118 are shown in the inner band 119a and the outer band 119 c. As the disk 10 rotates about its center 13in the direction of arrow 20, the movement of actuator 130 allows theread/write head on the trailing end of head carrier 122 to accessdifferent data tracks 118 on disk 10. Rotation of the actuator 130 aboutpivot 132 to cause the read/write head on the trailing end of headcarrier 122 to move from near the disk inside diameter (ID) to near thedisk outside diameter (OD) will result in the read/write head making anarcuate path across the disk 10.

FIG. 2 is a top view of an enlarged portion of disk 10 showing thedetailed arrangement of the data islands 30 in one of the bands on thesurface of disk substrate 11 according to the prior art. While theislands 30 are shown as being circularly shaped, they may have othershapes, such as generally rectangularly or generally elliptical. Theislands 30 contain magnetizable recording material and are arranged intracks spaced-apart in the radial or cross-track direction, as shown bytracks 118 a-118 e. The tracks are typically spaced apart by a nearlyfixed track pitch or spacing TS. Within each track 118 a-118 e, theislands 30 are roughly equally spaced apart by a nearly fixedalong-the-track island pitch or spacing IS, as shown by typical islands30 a, 30 b, where IS is the spacing between the centers of two adjacentislands in a track. In FIG. 2, TS and IS are depicted as being equal, sothe BAR is 1. The islands 30 are also arranged into generally radiallines, as shown by radial lines 129 a, 129 b and 129 c that extend fromdisk center 13 (FIG. 1). Because FIG. 2 shows only a very small portionof the disk substrate 11 with only a few of the data islands, thepattern of islands 30 appears to be two sets of perpendicular lines.However, tracks 118 a-118 e are concentric rings centered about thecenter 13 of disk 10 and the lines 129 a, 129 b, 129 c are not parallellines, but radial lines extending from the center 13 of disk 10. Thusthe angular spacing between adjacent islands as measured from the center13 of the disk for adjacent islands in lines 129 a and 129 b in aradially inner track (like track 118 e) is the same as the angularspacing for adjacent islands in lines 129 a and 129 b in a radiallyouter track (like track 118 a).

The generally radial lines (like lines 129 a, 129 b, 129 c) may beperfectly straight radial lines but are preferably arcs orarcuate-shaped radial lines that replicate the arcuate path of theread/write head on the rotary actuator. Such arcuate-shaped radial linesprovide a constant phase position of the data islands as the head sweepsacross the data tracks. There is a very small radial offset between theread head and the write head, so that the synchronization field used forwriting on a track is actually read from a different track. If theislands between the two tracks are in phase, which is the case if theradial lines are arcuate-shaped, then writing is greatly simplified.

Patterned-media disks like that shown in FIG. 2 may be longitudinalmagnetic recording disks, wherein the magnetization directions in themagnetizable recording material are parallel to or in the plane of therecording layer in the islands, or perpendicular magnetic recordingdisks, wherein the magnetization directions are perpendicular to orout-of-the-plane of the recording layer in the islands. To produce therequired magnetic isolation of the patterned data islands, the magneticmoment of the regions between the islands must be destroyed orsubstantially reduced to render these spaces essentially nonmagnetic.Patterned media may be fabricated by any of several known techniques. Inone type of patterned media, the data islands are elevated, spaced-apartpillars that extend above the disk substrate surface to define troughsor trenches on the substrate surface between the pillars. This type ofpatterned media is shown in the sectional view in FIG. 3. In this typeof patterned media the substrate 11 with a pre-etched pattern of pillars31 and trenches or regions between the pillars can be produced withrelatively low-cost, high volume nanoimprinting process using a mastertemplate or mold. The magnetic recording layer material is thendeposited over the entire surface of the pre-etched substrate to coverboth the ends of the pillars 31 and the trenches between the pillars 31,resulting in the data islands 30 of magnetic recording layer materialand trenches 32 of magnetic recording layer material. The trenches 32 ofrecording layer material may be spaced far enough from the read/writehead to not adversely affect reading or writing to the recording layermaterial in islands 30, or the trenches may be rendered nonmagnetic by“poisoning” with a material like Si. This type of patterned media isdescribed by Moritz et al., “Patterned Media Made From Pre-EtchedWafers: A Promising Route Toward Ultrahigh-Density Magnetic Recording”,IEEE Transactions on Magnetics, Vol. 38, No. 4, July 2002, pp.1731-1736.

FIG. 4 is a schematic view of patterned-media disk 10 showing a patternof radial lines in three annular bands 119 a-119 c. Each radial line ismeant to represent data islands from all the concentric tracks in theband. The circumferential density of the radial lines is similar in allthree bands, with the angular spacing of the lines being adjusted in thebands to have smaller angular spacing in the direction from the diskinside diameter (ID) to outside diameter (OD), so that thecircumferential density of the radial lines, and thus the “linear” oralong-the-track density of data islands, stays relatively constant overall the bands on the disk. In actuality, a typical disk is divided intoabout 20 annular bands, which allows the linear density to remainconstant to within a few percent across all bands. Within each band, theradial lines are subdivided (not shown) into very short radial segmentsor lengths arranged in concentric rings, with each ring being a datatrack and each radial segment or length being a discrete data island.Each annular band, like band 119 c, has a band ID and a band OD. Also,in actuality the generally radial lines are more typically generallyarcuate lines that replicate the path of the read/write head mounted onthe end of the rotary actuator.

The making of the master template or mold to achieve an ultrahighdensity patterned-media disk is a difficult and challenging process. Theuse of electron beam (e-beam) lithography using a Gaussian beamrotary-stage e-beam writer is viewed as a possible method to make themaster mold. However, to achieve patterned-media disks with both higherareal bit density (greater than 1 Tbit/in²) and a higher BAR, a trackpitch of about 50 nm and an island pitch of about 12.5 nm will berequired, which would result in a BAR of 4. A master mold capable ofnanoimprinting patterned-media disks with an island pitch of 12.5 nm isdifficult to fabricate due to the limited resolution of e-beamlithography. Further increases in areal density will require evensmaller and denser features. For example, an areal density of 5 Tb/in²with a BAR of 2 will require an island pitch along-the-track of 8 nm.

The present invention relates to a method for making a master mold thatis used in the nanoimprinting process to make patterned-media disks withan island pitch difficult to achieve with the resolution of e-beamlithography, thus enabling both higher areal bit density (1 Tbit/in² andhigher) and a high BAR (greater than 1, preferably equal to or greaterthan 2). The master mold may be used to directly nanoimprint the disks,but more likely is used to make replica molds which are then used todirectly nanoimprint the disks. The method uses conventional or e-beamlithography to form a pattern of generally radial stripes on asubstrate, with the stripes being grouped into annular zones or bands. Ablock copolymer material is deposited on the pattern, resulting inguided self-assembly of the block copolymer into its components tomultiply the generally radial stripes into generally radial lines. Theradial lines preferably have a higher circumferential density than thatof the radial stripes. Various methods, including conventionallithography, guided self-assembly of a second block copolymer, ande-beam lithography, are then used to form concentric rings over thegenerally radial lines. The concentric rings are used to define theradial length of the islands formed by the master mold. After etchingand resist removal, the master mold has a pattern of either pillars orholes, depending on the method used. The pillars or holes are arrangedin circular rings, with the rings grouped into annular bands. Thespacing of the concentric rings is selected so that following theetching process the master mold has an array of pillars or holes withthe desired BAR, which is greater than 1, preferably about 2 or greater.Because the invention allows the circumferential density of the mastermold pillars or holes to be at least doubled from what could be achievedwith just e-beam lithography, the subsequently nanoimprintedpatterned-media disks can have both a high BAR (greater than 1 andpreferably equal to or greater than 2) and an ultra-high areal density.

Self-assembling block copolymers have been proposed for creatingperiodic nanometer (nm) scale features. A self-assembling blockcopolymer typically contains two or more different polymeric blockcomponents, for example components A and B, that are immiscible with oneanother. Under suitable conditions, the two or more immiscible polymericblock components separate into two or more different phases ormicrodomains on a nanometer scale and thereby form ordered patterns ofisolated nano-sized structural units. There are many types of blockcopolymers that can be used for forming the self-assembled periodicpatterns. If one of the components A or B is selectively removablewithout having to remove the other, then an orderly arranged structuralunits of the un-removed component can be formed. There are numerousreferences describing self-assembling block copolymers, including U.S.Pat. No. 7,347,953 B2; Kim et al., “Rapid Directed Self-Assembly ofLamellar Microdomains from a Block Copolymer Containing Hybrid”, Proc.of SPIE Vol. 6921, 692129, (2008); Kim et al., “Device-Oriented DirectedSelf-Assembly of Lamella Microdomains from a Block Copolymer ContainingHybrid”, Proc. of SPIE Vol. 6921, 69212B, (2008); and Kim et al.,“Self-Aligned, Self-Assembled Organosilicate Line Patterns of ˜20 nmHalf-Pitch from Block Copolymer Mediated Self-Assembly”, Proc. of SPIEVol. 6519, 65191H, (2007).

Specific examples of suitable block copolymers that can be used forforming the self-assembled periodic patterns include, but are notlimited to: poly(styrene-block-methyl methacrylate) (PS-b-PMMA),poly(ethylene oxide-block-isoprene) (PEO-b-PI), poly(ethyleneoxide-block-butadiene) (PEO-b-PBD), poly(ethylene oxide-block-styrene)(PEO-b-PS), poly(ethylene oxide-block-methylmethacrylate) (PEO-b-PMMA),poly(ethyleneoxide-block-ethylethylene) (PEO-b-PEE),poly(styrene-block-vinylpyridine) (PS-b-PVP),poly(styrene-block-isoprene) (PS-b-PI), poly(styrene-block-butadiene)(PS-b-PBD), poly(styrene-block-ferrocenyldimethylsilane) (PS-b-PFS),poly(butadiene-block-vinylpyridine) (PBD-b-PVP),poly(isoprene-block-methyl methacrylate) (PI-b-PMMA), andpoly(styrene-block-dymethylsiloxane) (PS-b-PDMS).

The specific self-assembled periodic patterns formed by the blockcopolymer are determined by the molecular volume ratio between the firstand second polymeric block components A and B. When the ratio of themolecular volume of the second polymeric block component B over themolecular volume of the first polymeric block component A is less thanabout 80:20 but greater than about 60:40, the block copolymer will forman ordered array of cylinders composed of the first polymeric blockcomponent A in a matrix composed of the second polymeric block componentB. When the ratio of the molecular volume of the first polymeric blockcomponent A over the molecular volume of the second polymeric blockcomponent B is less than about 60:40 but is greater than about 40:60,the block copolymer will form alternating lamellae composed of the firstand second polymeric block components A and B. In the present invention,the un-removed component is to be used as an etch mask, so orderedarrays of alternating lamellae and alternating cylinders are ofinterest.

The periodicity or bulk period (L₀) of the repeating structural units inthe periodic pattern is determined by intrinsic polymeric propertiessuch as the degree of polymerization N and the Flory-Huggins interactionparameter χ. L₀ scales with the degree of polymerization N, which inturn correlates with the molecular weight M. Therefore, by adjusting thetotal molecular weight of the block copolymer of the present invention,the bulk period (L₀) of the repeating structural units can be selected.

To form the self-assembled periodic patterns, the block copolymer isfirst dissolved in a suitable solvent system to form a block copolymersolution, which is then applied onto a surface to form a thin blockcopolymer layer, followed by annealing of the thin block copolymerlayer, which causes phase separation between the different polymericblock components contained in the block copolymer. The solvent systemused for dissolving the block copolymer and forming the block copolymersolution may comprise any suitable solvent, including, but not limitedto: toluene, propylene glycol monomethyl ether acetate (PGMEA),propylene glycol monomethyl ether (PGME), and acetone. The blockcopolymer solution can be applied to the substrate surface by anysuitable techniques, including, but not limited to: spin casting,coating, spraying, ink coating, dip coating, etc. Preferably, the blockcopolymer solution is spin cast onto the substrate surface to form athin block copolymer layer. After application of the thin blockcopolymer layer onto the substrate surface, the entire substrate isannealed to effectuate microphase segregation of the different blockcomponents contained by the block copolymer, thereby forming theperiodic patterns with repeating structural units.

The block copolymer films in the above-described techniquesself-assemble without any direction or guidance. This undirectedself-assembly results in patterns with defects so it is not practicalfor applications that require long-range ordering, such as for makingannular bands of radial lines on a master mold for nanoimprintingpatterned-media disks.

Lithographically patterned surfaces have been proposed to guide ordirect the self-assembly of block copolymer domains. One approach usesinterferometric lithography to achieve ordering of the domains withregistration of the underlying chemical contrast pattern on thesubstrate. Lamellar and cylindrical domains may be formed on a substrateby this technique, as described in U.S. Pat. No. 6,746,825. However,interferometric lithography cannot be used to make annular bands ofradial lines. US 2006/0134556 A1 describes techniques for creating achemical contrast pattern to guide the self-assembly of block copolymersto form aperiodic patterns. Also, in both of these approaches to createchemical contrast patterns on the substrate to guide the self-assemblyof block copolymers, the periodicity of the underlying chemical contrastpattern matches the bulk period L₀ of the block copolymer. For example,in US 2006/0134556 A1, L₀ is about 40 nm, so thelithographically-patterned substrate used to guide the self-assemblyalso has a period of about 40 nm, which can be achieved by conventionalor e-beam lithography. However, it is difficult to use conventional ore-beam lithography to create a chemical contrast pattern for a blockcopolymer with L₀ between about 8 nm and 30 nm.

First Embodiment

A first embodiment of the method of this invention for making the mastermold uses conventional optical or e-beam lithography to form a patternof generally radial stripes on a substrate, with the stripes beinggrouped into annular zones or bands. A block copolymer material isdeposited on the pattern, resulting in guided self-assembly of the blockcopolymer into its components to multiply the generally radial stripesinto generally radial lines of alternating block copolymer components.The radial lines of one of the components are removed, leaving theradial lines of the remaining component to be used as an etch mask toetch the substrate. A protective layer is then deposited over the radiallines of the remaining component to prevent their movement duringsubsequent processing. Conventional lithography is then used to formconcentric rings over the generally radial lines of the remainingcomponent. After etching to remove portions of the remaining blockcopolymer component between the concentric rings, and removal of theresist, a pattern of pillars of the remaining block copolymer componentis formed on the substrate. These pillars of remaining block copolymercomponent are used as an etch mask to pattern the substrate. Afteretching and removal of the pillars of remaining block copolymercomponent, a master mold remains that has pillars of substrate materialarranged in circular rings, with the rings grouped into annular bands.The spacing of the concentric rings is selected so that the master moldhas an array of pillars with the desired BAR, which is greater than 1,preferably about 2 or greater. The master mold may be used to directlynanoimprint the disks, but more likely is used to make replica moldswhich are then used to directly nanoimprint the disks.

The first embodiment of the method is explained with respect to FIGS.5A-5N. FIGS. 5A-5C , 5E-5G and 5I are side sectional views, at variousstages of the fabrication method, taken through a plane generallyperpendicular to the radial direction, and FIGS. 5D, 5H and 5J-5N aretop views at various stages of the first embodiment of the method.

In this first embodiment of the method, as shown in FIG. 5A, the mastermold substrate comprises a base 200, which may be formed of any suitablematerial, such as, but not limited to, single-crystal Si, amorphous Si,silica, quartz, silicon nitride, carbon, tantalum, molybdenum, chromium,alumina and sapphire. A nearly neutral layer 205 of a material that doesnot show a strong wetting affinity by one of the polymer blocks over theother, that will be referred to as “neutral layer”, is deposited ontothe base 200. The neutral layer can be, but is not restricted to, afunctionalized polymer brush, a cross-linkable polymer, a functionalizedpolymer “A” or “B” or a functionalized random copolymer “A-r-B” or ablend of “A” and “B”, where “A” and “B” are the constituent blockmaterials of the block copolymer. The functional group may be, forexample, a hydroxyl group. In the present example, the neutral layer 205is a hydroxyl-terminated polystyrene brush of lower molecular weightthan the block copolymer used. The brush material is spin-coated on base200 to a thickness of about 1-10 nm (below 6 nm is preferred). Thepurpose of the neutral layer is to tune the surface energy adequately topromote the desired domain orientation (perpendicular lamellae orparallel cylinders) and to provide the adequate wetting conditions fordensity multiplication.

In FIG. 5B a resist layer has been deposited on brush layer 205 andpatterned into generally radial bars 210 of resist. The resist layer ispatterned by e-beam and developed to form the pattern of radial bars 210separated by radial spaces 211 that expose portions of brush layer 205.The e-beam tool patterns the resist layer so that the radial spaces 211have a circumferential spacing that is approximately an integer multipleof L₀ (i.e., nL₀), the known bulk period for the selected blockcopolymer that will be subsequently deposited. In FIG. 5B, n is 2. Thecircumferential width of each radial space 211 is selected to beapproximately 0.5 L₀.

In FIG. 5C, the structure is etched, by a process of oxygen plasmareactive ion etching (O₂ RIE), to remove portions of brush layer 205 inthe radial spaces 211, which exposes portions of base 200.Alternatively, the chemical structure of the exposed portions of brushlayer 205 in the radial spaces 211 can be chemically altered (by oxygenplasma etching or other process such as reactive ion etching, neutralatom (such as Ar) or molecule milling, ion bombardment andphotodegradation) so that the exposed portions of brush layer 205 have apreferred affinity for one of the copolymers. In FIG. 5D, which is a topview, the resist 210 is removed, leaving on the substrate 200 a patternof generally radial bars 205 of polymer brush material separated bygenerally radial stripes 200 of base material (or chemically-alteredbrush material). In this pattern the generally radial stripes 200 have acircumferential width of 0.5 L₀ and a circumferential pitch of 2 L₀.Because FIG. 5D is only a very small portion of the master mold, thestripes 200 appear as parallel stripes. However, the stripes 200 arearranged generally radially, as depicted in FIG. 4. The stripes 200 maybe perfectly straight radial stripes but are preferably arcs orarcuate-shaped radial stripes that replicate the arcuate path of theread/write head on the rotary actuator.

Next, in FIG. 5E, a layer 220 of block copolymer material is depositedover the radial bars 205 of brush material and the radial stripes 200 ofbase material (or chemically-altered brush material) in the radialspaces 211. The preferred block copolymer material is the diblockcopolymer polystyrene-block-polymethylmethacrylate (PS-b-PMMA) with L₀between about 8 nm and 30 nm and is deposited by spin coating to athickness of about 0.5 L₀ to 3 L₀.

In FIG. 5F, the block copolymer layer has been annealed, for example byheating to about 200 deg. C for approximately 60 minutes, which resultsin phase separation between the different components contained in theblock copolymer. In this example, the B component (PMMA) has an affinityfor the surface of base 200 or for the polar groups of the chemicallyaltered brush 205 and thus form as generally radial lines 215 on top ofthe radial stripes 200. Because the circumferential width of the stripes200 is approximately 0.5 L₀, the A component (PS) form in adjacentradial lines 212 on the radial bars 205 of polymer brush material. As aresult of the self-assembly of the A and B components this causes the Bcomponent to also form as generally radial lines 215 on the centers ofeach radial bar 205 of polymer brush material. The generally radialstripes 200 (or chemically altered brush) thus guide the self-assemblyof the PS and PMMA components to form the alternating radial lines 212,215 in the structure as shown in FIG. 5F. Although the A and Bcomponents prefer to self-assemble in parallel lines with a period ofL₀, the substrate pattern of radial stripes 200 guides the alternatinglines 212, 215 to form as radial lines, which means that that L₀ cannotbe constant over the entire radial length. However, a pattern ofalternating radial lines 212, 215 can be accomplished without anysignificant defects if the variation from L₀ does not exceedapproximately 10 percent. Thus, to achieve this, the circumferentialspacing of the radial stripes 200 at the band ID should not be less thanabout 0.9 nL₀ and the circumferential spacing of the radial stripes 200at the band OD should not be greater than about 1.1 nL₀.

Next, in FIG. 5G, the B component (PMMA) is selectively removed by a wetetch (acetic acid, IPA or other selective solvent) or a dry etch process(O₂ RIE), leaving generally radial lines 212 of the A component (PS).FIG. 5H is a top view of FIG. 5G and shows the generally radialA-component lines 212 with a circumferential spacing L₀. In FIG. 5H thecircumferential density of radial lines 212 has been doubled from thecircumferential density of radial stripes 200 in FIG. 5D.

After the radial lines 212 have been formed as shown in FIG. 5H, theyare cut into circumferential segments or rings that will correspond tothe tracks on the patterned-media disks that will be nanoimprinted bythe master mold. The first step in this part of the method is shown inthe side sectional view of FIG. 5I in which a protective layer 206 isdeposited over the structure of FIG. 5H. The protective layer 206 may beSi, SiO₂, alumina (Al₂O₃) or similar material sputter deposited to athickness of approximately 1-2 nm. The purpose of the protective layer206 is to prevent movement and/or dissolving of the radial lines 212during subsequent processing. Next, in the top view of FIG. 5J, a layerof e-beam resist 217 is deposited over the protective layer 206 and theresist 217 is exposed in a rotary-stage e-beam tool to expose narrowconcentric boundary regions 207 that correspond to the boundariesbetween the tracks of the patterned-media disks to be nanoimprinted. Theconcentric boundary regions 207 are spaced about by a distance greaterthat L₀ and preferably greater than 2 L₀, so that the BAR is greaterthan 1 and preferably equal to or greater than 2. The boundary regions207 are portions of alternating radial lines 212 and radial stripes 200covered with protective layer 206. The resist 217 may be a positivee-beam resist like poly methyl methacrylate (PMMA) or ZEP520 from ZeonChemicals, L.P. After developing, this will leave circumferentialsegments 213, which correspond to the tracks on the patterned-mediadisks to be nanoimprinted, covered with resist 217, with the boundaryregions 207 between tracks not covered with resist, but covered withprotective layer 206. By adjusting the exposure and developingconditions, the width of the uncovered boundary regions can be adjustedas desired.

In FIG. 5K, the protective layer 206 is removed by fluorine-based RIE,for example RIE with CHF₃. This leaves the boundary regions 207 asportions of alternating radial lines 212 and radial stripes 200 notcovered with protective layer 206. In FIG. 5L, the PS (block copolymercomponent A) in the exposed portions of radial lines 212 in the boundaryregions 207 is removed by a O₂ RIE process. Then, in FIG. 5M the resist217 is removed in a wet etch process, like hot NMP. This leaves pillars226 of PS (covered with protective layer 206) arranged incircumferential segments 213 on substrate 200.

Then, in FIG. 5N, a dry etch process is used to etch the substrate 200in the regions between the PS pillars 226, using the PS pillars 226 asan etch mask. The PS pillars 226 are then removed by a O₂ RIE process,leaving pillars 228 of substrate material on substrate 200. This leavesthe structure as shown in FIG. 5N with the pillars 228 being arranged incircumferential segments 213 which correspond to the concentric tracksof the patterned-media disks to be nanoimprinted. The structure of FIG.5N, which began as a substrate of base 200, has now been etched so thata portion of the substrate remains as the topographic pattern in theform of pillars 218. The structure of FIG. 5N can function as the mastermold with the pillars 218 functioning as the topographic pattern fornanoimprinting the replica molds.

FIG. 5O is a scanning electron microscope (SEM) micrograph of a portionof the master mold made according to the method shown in FIGS. 5A-5N.The white generally rectangular images are pillars 218. The linear oralong-the-track island or bit pitch is 27 nm and the track pitch is 54nm for a BAR of 2. Each pillar has a radial length of about 40 nm and acircumferential width of about 16 nm for an IAR of about 2.5. Thisresults in a bit cell area of 1458 nm² (27 nm×54 nm) and a pillar sizeor surface area of 640 nm² (40 nm×16 nm). Thus in the resulting diskmade from this mold an island would occupy about 44% of the bit cell(640/1458) and the areal density would be about 0.44 Tbits/in².

The SEM image for the entire sample of the master mold in FIG. 5O wasprocessed, using well-known image processing algorithms, to measure thesize of each pillar and the actual placement of the centroid of eachpillar. This image was then compared with an ideal lattice for thedesired pillar size and placement, i.e., 640 nm² pillars with a 27 nmbit pitch and a 54 nm track pitch. It was determined that the standarddeviation of the area of the pillars was less than 10 percent, and thestandard deviation of the placement of the pillars in both thealong-the-track and cross-track directions was less than 5 percent.Also, the number of defective pillars was less than one in 10⁵. Theinvention thus enables patterned-media disks to be nanoimprinted withuniform feature size and uniform feature placement over the entiresurfaces of the disks.

In the method for making the master mold shown in the SEM micrograph ofFIG. 5O, the protective layer 206 (FIG. 5I) was a sputter-deposited 2 nmthick film of SiO₂. The protective film 206 prevented movement anddissolving of the underlying radial lines 212 and thus enabled the smallclosely-spaced features of pillars 228. With the method of thisinvention it is possible to fabricate a master mold with featuressmaller and more closely-spaced than as shown in FIG. 5O and with thesame accuracy in area and placement as measured from the SEM image ofFIG. 5O. Specifically a master mold can be fabricated with pillarshaving a bit pitch of 13 nm, a track pitch of 26 nm and with dimensionsof about 7 nm by 13 nm (91 nm²). Thus disks can be fabricated withislands having this uniform size and uniform spacing. The islands wouldoccupy about 27% of the bit cell, resulting in an areal density of about2 Tbits/in². The polymers that can be used are any of those describedabove. The achievable areal density is limited by the resolution of thee-beam resist used to define the circumferential lines or rings (theminimum achievable track pitch for the circumferential rings is about25-35 nm). The radial lines can be made to have a smallercircumferential width and can be made more closely spaced. To keep theBAR at about 2, the island pitch is selected to be 13 nm. However,higher areal densities using the method of the first embodiment can beachieved by using a higher BAR where the track pitch is still about 26nm, but the island pitch is smaller by use of a polymer with a smallerbulk period L₀ and by use of a higher multiplication factor, i.e., theradial lines can be pre-patterned at a spacing of 3 L₀ or 4 L₀ insteadof 2 L₀.

A variation of the method of the first embodiment uses a negativeresist, such as hydrogen silsesquioxane (HSQ) or calixarene to form acircumferential network of rings bridging the radial lines, rather thanresist grooves as in FIGS. 5J and 5K. In this case, surface layers ofthe substrate 200 are patterned as done in the first embodiment, asshown up to FIG. 5I. The substrate 200 with copolymer radial lines 212and protective layer 206, as shown in FIG. 5I, is then coated with anegative resist. The e-beam writer then writes the same pattern ofcircumferential rings, as described in the first embodiment. In contrastto the first embodiment, FIGS. 5J and 5K, the e-beam exposed resistrings are insoluble in the developer. The unexposed regions of resistare soluble in the developer. This leaves a grid of concentric resistrings, instead of the concentric boundary regions 207 in FIGS. 5J and5K, overlapping the radial lines 212. The substrate is then etchedthrough this grid, which acts as an etch mask. The resist in theconcentric rings, the material of protective overcoat 206, and the blockcopolymer material in radial lines 212 are then removed. This leaves astructure that appears like that of FIG. 5N, except that the features228 are not pillars but are holes or recesses in the substrate 200.

Second Embodiment

A second embodiment of the method of this invention for making themaster mold uses conventional optical or e-beam lithography to form apattern of generally radial stripes on a substrate, with the stripesbeing grouped into annular zones or bands. A first block copolymermaterial with bulk period L₀ =L_(rad) is deposited on the pattern,resulting in guided self-assembly of the block copolymer into itscomponents to multiply the generally radial stripes into generallyradial lines of alternating block copolymer components. The radial linesof one of the components are removed, leaving the radial lines of theremaining component of the first block copolymer. A protective layer isthen deposited over the radial lines of the remaining component of thefirst block copolymer to prevent their movement during subsequentprocessing. Then, a second block copolymer material with bulk period L₀=L_(circ) is deposited over these radial lines to define generallycircumferential rings. The circumferential rings of one of thecomponents of the second block copolymer are removed, leaving thecircumferential rings of the remaining component of the second blockcopolymer. The circumferential rings of the remaining second blockcopolymer component and the underlying radial lines of the remainingfirst block copolymer component form a grid that functions as an etchmask. Etching of the substrate through this mask, followed by removal ofthe remaining block copolymer material, results in a master mold with apattern of recesses or holes arranged in circular rings, with the ringsgrouped into annular bands. The ratio of L_(circ)/L_(rad) defines theBAR for the disk made from the mold.

The second embodiment of the method is explained with respect to FIGS.6A-6H. FIGS. 6A-6B are side sectional views, at various stages of thefabrication method, taken through a plane generally perpendicular to theradial direction, and FIGS. 6C-6H are top views at various stages of themethod.

The second embodiment of the method begins with the structure of FIG. 6Awhich is identical to FIG. 5I and shows the protective layer 206 formedover radial lines 212 of the A component of the first block copolymermaterial. The radial lines 212 are circumferentially spaced-apart by adistance L_(rad), where L_(rad) is close to the bulk period of the firstblock copolymer material. In FIG. 6B a surface modification or neutrallayer 230, like neutral layer 205 used in the first embodiment, isapplied over the protective layer 206. Next, in FIG. 6C an e-beam resistfilm 317 is applied over the surface modification layer 230 andpatterned into circumferential rings 313. The resist layer 317 ispatterned by e-beam and developed to form the pattern of circumferentialrings 313 separated by concentric boundary regions 307 that correspondto the boundaries between the tracks of the patterned-media disks to benanoimprinted. The concentric regions 307 expose alternating portions ofthe substrate 200 and portions of previously formed radial lines 212,which are covered with surface modification layer 230. The e-beam writerpatterns the resist layer 317 so that the concentric regions 307 areradially spaced-apart by a distance nL_(circ), where n is an integer andL_(circ) is the bulk period of the second block copolymer material thatwill be subsequently deposited. In FIG. 6C, n=2 and the radial length ofthe concentric regions 307 is 0.5 L_(circ.) Also, in this example,L_(circ) is chosen to be 2 L_(rad), as depicted in FIG. 6C. In FIG. 6D,the exposed portions (regions 307) are etched or chemically altered byoxygen plasma etching (or other process such as reactive ion etching,neutral atom or molecule milling, ion bombardment and photodegradation)to remove or chemically alter the composition of the surfacemodification layer 230, as represented by etched or altered layer 230′in regions 307. In FIG. 6E, the e-beam resist is removed by use of asuitable solvent, leaving circumferential rings 313 of alternatingradial lines 200 and 212 covered with surface modification layer 230 andconcentric regions 307 of alternating portions of the substrate 200 andportions of previously formed radial lines 212 covered with the alteredlayer 230′.

In FIG. 6F, the second block copolymer material is deposited over theradial lines 212 covered with brush material 230 and the concentricboundary regions 307, which are covered with the altered layer 230′. Thepreferred second block copolymer material may also be the diblockcopolymer polystyrene-block-polymethylmethacrylate (PS-b-PMMA). Thecharacteristic bulk period, L₀, in a block copolymer is determined byits degree of polymerization, N, i.e., the number of constituentmonomers in the polymer chain length. Block copolymers with differentvalues of L₀ can be chosen by selecting the appropriate molecularweights. For example, a symmetric PS-b-PMMA with a total molecularweight of Mw=46 Kg/mol displays an L₀ of approximately 32 nm whereas onewith Mw=36 Kg/mol exhibits an L₀ of approximately 27 nm. Other valuesfor L₀ are known and described by Black, C. T., Ruiz, R., et al.,“Polymer self assembly in semiconductor microelectronics”, IBM Journalof Research and Development, Volume 51, Number 5, Page 605 (2007). Inthis second embodiment L₀ for the first block copolymer equals L_(rad)and L₀ for the second block copolymer material equals L_(circ), withL_(rad) and L_(circ) being chosen according to the desired areal densityand bit aspect ratio (BAR). For a BAR of approximately 2, L_(circ)=2L_(rad), as shown in the example depicted in FIGS. 6A-6E. In FIG. 6F,the second block copolymer layer has been annealed, which results inphase separation between the different components contained in the blockcopolymer. In this example, the B component (PMMA) has an affinity forthe chemically altered brush 230 in boundary regions 307 and thus formas generally circumferential rings 318. Because the radial width of theregions 307 is approximately 0.5 L₀, the A component (PS) form inadjacent circumferential rings 319. As a result of the self-assembly ofthe A and B components this causes the B component to also form asgenerally circumferential rings 318 between the A component rings 319with radial spacing L_(circ).

In FIG. 6G, the B component (PMMA) is selectively removed, for example,by use of ultraviolet (UV) radiation followed by a rinse in a selectivesolvent, as described by Thurn-Albrecht, T. et al., “NanoscopicTemplates from Oriented Block Copolymer Films”, Advanced Materials 2000,12, 787. Then the remains of the surface modification layer 230 areremoved, leaving circumferential rings of portions of alternating radiallines 212 (the A component of the first block copolymer) and 200 (thesubstrate). The resulting structure in FIG. 6G is a grid ofcircumferential rings 318 of the A component (PS) of the second blockcopolymer A and underlying radial lines 212 of the A component (PS) ofthe first block copolymer A. This grid defines and exposes generallyrectangular regions 200 of substrate material. The circumferential pitchof the radial lines 212 is defined by the periodicity of the first blockcopolymer film, while the radial pitch of rings 319 is defined by theperiodicity of the second block copolymer. In this method where bothradial lines 212 and circumferential rings 319 of PS material aredefined by guided self-assembly of block copolymers, the order of thefabrication process may be reversed, i.e., the circumferential rings 319may be defined first followed by the assembly of the radial lines 212,which would then be located above the underling circumferential rings319.

Then, in FIG. 6H, a dry etch process is used to etch the substrate 200to form recesses or holes 229, using the grid of intersectingcircumferential rings 319 and radial lines 212 as the etch mask. The PSmaterial of circumferential rings 319 and underlying radial lines 212 isthen removed by a O₂ RIE process, leaving holes 229 in substrate 200.This leaves the structure as shown in FIG. 6H with the holes 229 beingarranged in circumferential segments 323 which correspond to theconcentric tracks of the patterned-media disks to be nanoimprinted. Theresulting disk will have a linear or along-the-track bit pitch ofL_(rad) and a track pitch of about L_(circ). In the example of FIG. 6H,L_(circ)=2 L_(rad), for a BAR of about 2. The structure of FIG. 6H,which began as a substrate of base 200, has now been etched to definethe pattern of holes 229 below the original surface of substratematerial 200. The structure of FIG. 6H can function as the master moldwith the holes 229 functioning as the topographic pattern fornanoimprinting the replica molds.

Third Embodiment

A third embodiment of the method of this invention for making the mastermold uses conventional optical or e-beam lithography to form a patternof generally radial stripes on a substrate, with the stripes beinggrouped into annular zones or bands. A block copolymer material withbulk period L₀ is deposited on the pattern, resulting in guidedself-assembly of the block copolymer into its components to multiply thegenerally radial stripes into generally radial lines of alternatingblock copolymer A and B components, as described in the firstembodiment. Then an e-beam writer generates a high dose e-beam in apattern of concentric rings which cross-links the A and B copolymersexposed to the high dose e-beam, resulting in concentric rings formed ofcross-linked polymer material. The underlying radial lines of the Bcomponent are then removed, leaving the cross-linked concentric ringsand underlying radial lines of the A component. This structure thenserves as an etch mask to pattern recesses or holes into the underlyingsubstrate.

The third embodiment of the method is explained with respect to FIGS.7A-7D. The third embodiment of the method begins with the structure ofFIG. 7A, which is identical to FIG. 5F and which is a side sectionalview showing alternating A component (PS) radial lines 212 and Bcomponent (PMMA) radial lines 215. In FIG. 7B, a top view, a high doseof e-beam radiation is used to define circumferential rings 407 betweencircumferential rings 413 of alternating radial lines 212 and 215. Thecircumferential rings 407 have center-to-center radial spacing equal tothe desired track pitch of the disks made to be made with the mastermold. The circumferential rings 407 will ultimately result in theboundary regions between the tracks of the disks made with the mastermold. The e-beam radiation dose is selected to cause both the A and Bcopolymers to cross-link, resulting in hardened, non-solublecircumferential rings 407. Polystyrene (PS) is well known as a crosslinking negative resist. While PMMA is normally a positive resist (theexposed area is subsequently removed and the unexposed area remains), itcan be used as a negative resist (the exposed area hardens and theunexposed area is later removed) by overexposure with e-beam dosesranging from about 10 to 100 times the dosage normally used when usingPMMA as a positive resist. See Dobisz, E. A. et al., “THINSILICON-NITRIDE FILMS FOR REDUCTION OF LINEWIDTH AND PROXIMITY EFFECTSIN ELECTRON-BEAM LITHOGRAPHY”, JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B,Vol 10, Issue 6, NOV-DEC 1992, pp. 3067-3071. The required e-beam dosagedepends on a number of factors, such as molecular weight of the resist,energy of the electrons, type of substrate, density of written features,and development process. In the example depicted in FIG. 7B, the blockcopolymer material was 36 Kg/mol (with the PMMA component being 18Kg/mol) and the e-beam line-dose to cross-link it was 2.8 nanoCoulombsper cm (nC/cm). In FIG. 7C, the entire film is subjected to UV-radiationthat cross-links the exposed portions of the PS radial lines 212 andscissions the exposed portions of the PMMA radial lines 215. Theportions of radial lines 215, the B component (PMMA), are thenselectively removed by a wet etch (acetic acid, IPA or other selectivesolvent) or a dry etch process (O₂ RIE). This leaves generally radiallines 212 of the A component (PS) beneath the circumferential rings 407.The resulting structure in FIG. 7C is a grid of intersectingcircumferential rings 407 of cross-linked copolymers and underlyingradial lines 212 of the block copolymer A component (PS), with exposedholes 200 of substrate material.

Then, in FIG. 7D, a dry etch process is used to etch the substrate 200to form recesses or holes 329, using the grid of intersectingcircumferential rings 407 and radial lines 212 as the etch mask. Thematerial of circumferential rings 407 and underlying radial lines 212 isthen removed by a O₂ RIE process, leaving holes 329 in substrate 200.This leaves the structure as shown in FIG. 7D, which is identical toFIG. 5N, with the holes 329 being arranged in circumferential segments423 which correspond to the concentric tracks of the patterned-mediadisks to be nanoimprinted. The structure of FIG. 7D, which began as asubstrate of base 200, has now been etched to define the pattern ofholes 329 below the original surface of substrate material 200. Thestructure of FIG. 7D can function as the master mold with the holes 329functioning as the topographic pattern for nanoimprinting the replicamolds. The along-the-track pitch of the holes 200 is defined by theperiodicity of the block copolymer film, while the track pitch of thecircumferential segments is defined by the e-beam writer.

In the embodiments of the method described above, the two blockcopolymer components are depicted as self-assembling into alternatinglamellae, as shown, for example, by alternating radial lines 212, 215 inFIG. 5F. For the A and B components (PS and PMMA) to form as alternatinglamellae the molecular weight ratio of the A to B components should bebetween about 40:60 and 60:40, preferably close to 50:50. However, it isalso within the scope of the invention for the A component (PS) to formas radially-aligned cylinders within a matrix of the B component (PMMA).To achieve this type of structure, wherein the A component cylindersform the radial lines 212 within alternating radial lines 215 of Bcomponent material, the molecular weight ratio of component B overcomponent A should be less than about 80:20 but greater than about60:40, preferably close to 70:30.

The master mold shown in FIG. 5N is a pillar-type master mold that canbe used to make replica molds. The replica molds will thus have holepatterns corresponding to the pillar pattern of the master mold. Whenthe replica mold is used to make the disks, the resulting disks willthen have a pillar pattern, with the pillars corresponding to the dataislands. The master molds shown in FIGS. 6H and 7D are hole-type mastermolds that can be used to directly nanoimprint the disks.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A master mold for imprinting magnetic recording disks comprising: amaster mold substrate; a plurality of substantially identical discretefeatures on the substrate, the features being arranged into a pluralityof concentric circular tracks and spaced-apart along the tracks, thefeatures having a uniform surface area less than or equal to 640 nm²with a standard deviation of said uniform surface area less than 10percent.
 2. The master mold of claim 1 wherein the concentric circulartracks are arranged into a plurality of annular zones, and wherein thefeatures have a uniform along-the-track spacing in at least one of saidzones of less than or equal to 27 nm, with a standard deviation of saidalong-the-track spacing of less than 5 percent.
 3. The master mold ofclaim 1 wherein the features in the concentric circular tracks have auniform cross-track spacing of less than or equal to 54 nm, with astandard deviation of said cross-track spacing of less than 5 percent.4. A patterned-media magnetic recording disk comprising: a disksubstrate; a plurality of substantially identical discrete magnetizablepillars on the substrate, the pillars being arranged into a plurality ofconcentric circular tracks and spaced-apart along the tracks, thepillars having a uniform surface area less than or equal to 640 nm² witha standard deviation of said uniform surface area less than 10 percent.5. The disk of claim 4 wherein the concentric circular tracks arearranged into a plurality of annular zones, and wherein the pillars inthe concentric circular tracks in at least one of said zones have auniform along-the-track spacing of less than or equal to 27 nm, with astandard deviation of said along-the-track spacing of less than 5percent.
 6. The disk of claim 4 wherein the pillars in the concentriccircular tracks have a uniform cross-track spacing of less than or equalto 54 nm, with a standard deviation of said cross-track spacing of lessthan 5 percent.
 7. A patterned-media magnetic recording disk comprising:a disk substrate; a plurality of substantially identical discretemagnetizable pillars on the substrate representing discrete data bits,the pillars being arranged into a plurality of concentric circulartracks and spaced-apart along the tracks, the pillars having an arealbit density greater than or equal to 1 Terabits/in² and a bit aspectratio (BAR) equal to or greater than 2, where BAR is the ratio of linearpillar density in bits per inch (BPI) in the along-the-track directionto track density in tracks per inch (TPI) in the cross-track direction.8. The disk according to claim 7 wherein the track pitch is greater thanor equal to 26 nm and less than or equal to 54 nm.
 9. The disk accordingto claim 7 wherein the bit pitch in the along-the-track direction isgreater than or equal to 13 nm and less than or equal to 27 nm.
 10. Amagnetic recording disk drive comprising: the disk according to claim 7;a write head for magnetizing the discrete pillars; a read head forreading the magnetized pillars; and an actuator connected to the writehead and read head for moving the write head and read head across thedisk to access the pillars.
 11. A magnetic recording disk drivecomprising: a patterned-media disk comprising a substrate and aplurality of substantially identical discrete magnetizable islands onthe substrate representing discrete data bits, the islands beingarranged into a plurality of concentric circular tracks with a trackpitch greater than or equal to 26 nm and less than or equal to 54 nm andspaced-apart along the tracks with an along-the-track bit pitch greaterthan or equal to 13 nm and less than or equal to 27 nm, the islandshaving an areal bit density greater than or equal to 1 Terabits/in² anda bit aspect ratio (BAR) equal to or greater than 2, where BAR is theratio of linear island density in bits per inch (BPI) in thealong-the-track direction to track density in tracks per inch (TPI) inthe cross-track direction; a write head for magnetizing the discreteislands; a read head for reading the magnetized islands; and an actuatorconnected to the write head and read head for moving the write head andread head across the disk to access the islands.